Abstract: Cyanobacteria separated from symbiosis with several species of the angiosperm Gunnera were comparatively characterized and correlated with the locales and taxonomy of their host plants. All were identified as strains of Nostoc . Protein profiles and DNA restriction fragment length polymorphisms (from hybridizations with heterologous nif H and gln A probes) determined that three of the four cyanobacteria from Gunnera grown at one site in Sweden, each from a different host species, were very similar or identical. Plants of one species, G. manicata , grown in a second location at the site were infected with a different cyanobiont. Among five isolates from two species of Gunnera , collected in the same locale in New Zealand, three subgroups were documented. Isolates from three different Gunnera species grown in separate locations in the United States were each uniquely different. None of the cyanobacteria differed in the molecular weights of their glutamine synthetase and Fe-nitrogenase proteins. The diversity and accessibility of compatible Nostoc populations present in the soil micro-environment, not a critical selective factor required by Gunnera , were concluded to be a major determinant in symbiont selection.